1. Field of the Invention
The present invention relates to a process for making sodium tripolyphosphate with reduced amounts of magnesium and silicon impurities from wet process phosphoric acid.
2. Description of the Prior Art
Sodium tripolyphosphate (sometimes referred to hereafter as STPP) is used today in many agricultural, industrial and food applications. Specifically, it has been utilized as a fertilizer component, as a builder in synthetic detergents and as an ingredient in many foodstuffs.
STPP has been normally made by a process that includes the steps of (a) reacting a sodium compound such as soda ash (Na.sub.2 CO.sub.3) or caustic (NaOH) with either wet process phosphoric acid or furnace process phosphoric acid to form a reaction mixture containing monosodium phosphate (NaH.sub.2 PO.sub.4) and disodium phosphate (Na.sub.4 HPO.sub.4); (b) precipitating and thereafter removing insoluble impurities from the reaction mixture; and (c) heating the reaction mixture at elevated temperatures for sufficient time to thermally decompose or calcinate the two above-named phosphate compounds into STPP (Na.sub.5 P.sub.3 O.sub.10) and to simultaneously remove water from the reaction mixture by volatilization. It should be noted that such processes removed impurities present in the STPP feedstock strictly by precipitation.
STPP made from wet process phosphoric acid is generally less costly than STPP made from furnace process phosphoric acid because the latter type requires large amounts of thermal energy in its production. But, STPP made from the wet process generally contains greater amounts of certain impurities, particularly silicon and magnesium, which may prevent this type of STPP from being employed in certain applications. Specifically, it has been found that the presence of silicon impurities, and to a lesser degree, magnesium impurities, often may cause turbidity or cloudiness in aqueous solutions containing STPP. Thus, industries, like the food industry, which prefer clear aqueous solutions of STPP may have to employ the more expensive STPP made from furnace process acid.
Accordingly, there is a need for a better process for making STPP from wet process acid which can reduce the amount of silicon and magnesium impurities in the product. As stated above, insoluble impurities in the STPP feedstock were removed solely by precipitation techniques. As an example, see Chemical Abstracts, Volume 80, page 17044s. However, it has been found that these strictly precipitation methods of purification are not efficient when the silicon and magnesium levels are each from below about 100 to about 800 parts per million (ppm) by weight of the feedstock. At such levels, their presence can still cause the undesirable turbidity, but their removal cannot be effected easily by precipitation without also losing valuable amounts of phosphate compounds. Thus, it is believed that an improved process for making STPP from wet process acid must remove silicon and magnesium impurities by some method other than strictly precipitation. The process of the present invention is directed towards such a method.